CASE ins.in_discount_ind
when 0 then
COALESCE(ins.in_principaldefaultamount, 0)
else
CASE ins.in_principaldefaultamount
when 0 then
NULL
else
ins.in_principaldefaultamount
end
end
AS [principal_amount_at_default]

You don't need to use a sub-select to execute a CASE function. Rewrite all this sub-selects to a CASE like I did above.
Sample:

I've changed all the CASE in your SELECT. This is the new code:
SELECT
ev.ev_id AS [access_default_id],
cl.cl_id AS [access_class_id],
ins.in_id AS [access_instrument_id],
ob.ob_moodysinternalid AS [moodys_org_id],
ins.in_moodysdebtid AS [moodys_debt_id],
ob.ob_name AS [name],
ob.ob_siccode AS [sic_code],
ob.ob_moodysindustrygrp AS [moodys_industry],
ev.ev_obligordefaultdate AS [date_of_obligor_default],
ev.ev_emergencedate AS [date_of_emergence],
ev.ev_defaulttype AS [default_type],
cl.cl_classtag AS [creditor_class_number],
cl.cl_classname AS [creditor_class_description],
ins.in_instrumenttype AS [instrument_type],
ins.in_instrumentdesc AS [instrument_description],
ins.in_collateralrank AS [instrument_ranking],
ins.in_collateraltype AS [collateral],
ins.in_maturitydate AS [maturity_date],
ins.in_originationdate AS [origination_date],
ins.in_originalamount AS [original_issuer_amount],
case ins.in_discount_ind
when 0 then
COALESCE(ins.in_principaldefaultamount, 0)
else
case ins.in_principaldefaultamount
when 0 then null else ins.in_principaldefaultamount end
end as [principal_amount_at_default],
ins.in_accretedamount AS [accreted_amount],
ev.ev_totaldebt AS [issuer_total_debt],
COALESCE(ins.in_debtabove, 0) as [principal_above_$],
COALESCE(ins.in_percentabove, 0) as [principal_above_%],
COALESCE(ins.in_debtbelow, 0) as [principal_below_$],
COALESCE(ins.in_percentbelow, 0) as [debt_cushion_%],
ins.in_CUSIP AS [cusip_number],
ins.in_defaultdate AS [date_of_instrument_default],
ins.in_lastdatecashpaid AS [last_date_interest_paid],
case ins.in_didnotdefault when 1 then 'Y' else 'N' end as [never_defaulted],
ins.in_interestbaserate AS [interest_rate_index],
ins.in_valueoverbaserate/100 AS [spread_over_index],
ins.in_effectiveinterestrate/100 AS [effective_interest_rate],
cl.cl_settlementdesc AS [treatment],
cl.cl_preferredmethod AS [recovery_method_recommended],
case cl.cl_preferredmethod
when 'Settlement'
then ins.in_nominalsettlementtotal
else
case cl.cl_preferredmethod
when 'Trading Price'
then ins.in_nominaltradingprice
else
case cl.cl_preferredmethod
when 'Liquidity'
then ins.in_nominalliquiditytotal
else Null
end
end
end /100 AS [recommended_nominal_recovery],
case cl.cl_preferredmethod
when 'Settlement'
then ins.in_discountsettlementtotal
else
case cl.cl_preferredmethod
when 'Trading Price'
then ins.in_discounttradingprice
else
case cl.cl_preferredmethod
when 'Liquidity'
then ins.in_discountliquiditytotal
else Null
end
end
end /100 AS [recommended_discounted_recovery],
case cl.cl_useTradingPrice when 1 then ins.in_nominaltradingprice/100 else Null end AS [trading_price_nominal_recovery],
case cl.cl_useTradingPrice when 1 then in_discounttradingprice/100 else Null end AS [trading_price_discounted_recovery],
case cl.cl_useSettlement when 1 then ins.in_nominalsettlementtotal/100 else Null end AS [settlement_nominal_recovery],
case cl.cl_useSettlement when 1 then ins.in_discountsettlementtotal/100 else Null end AS [settlement_discounted_recovery],
case cl.cl_useLiquidity when 1 then ins.in_nominalliquiditytotal/100 else Null end AS [liquidity_event_nominal_recovery],
case cl.cl_useLiquidity when 1 then ins.in_discountliquiditytotal/100 else Null end AS [liquidity_event_discounted_recovery],
ev.ev_familyrecovery/100 AS [family_recovery],
case ins.in_tradingprice30day when 0 then Null else ins.in_tradingprice30day end AS [30_day_trading_price],
case ins.in_tradingprice30day when 0 then Null else ins.in_tradingprice30daydate end AS [30_day_trading_date],
mp.aristeia_issuer_id AS [aristeia_issuer_id],
sv.sv_serial_number AS [capiq_serial_number],
P.product_id AS [product_id]
, (select top 1
value
from as13.hal_db_readonly.dbo.product_detail PE
where PE.aristeia_field_code = 'PRIMARY_RISK_ID '
and P.product_id = PE.product_id
and PE.source_id = 0
and PE.expiration_date > ev.ev_obligordefaultdate
order by PE.expiration_date
) AS [primary_risk_id]
FROM .......